Ab initio studies of S chemisorption on GaAs(100)

Ab initio molecular orbital calculations are used to study S chemisorption on reconstructed GaAs(100) surfaces. Three cluster models are developed to simulate the main characteristics of 4×2, 4×6, and 2×6 reconstructed GaAs(100) surfaces. S atoms chemisorb preferentially on bridge sites and dramatically change the surface geometry. If a single S is adsorbed on a bridge site by breaking a Ga—Ga dimer bond, the Ga–Ga separation increases to 4.10 Å (from the dimerized separation of 2.80 Å), with a concomitant lowering of the bond energy by 0.07 (hartree). When two S atoms are adsorbed on adjacent bridge sites, the two S atoms weakly repel each other (establishing themselves ∼10° off from the normal bridge position). At the equilibrium geometry, each S atom retains a charge of −0.6 (e) and increases the net dipole moment perpendicular to the surface by −1.12 (hartree). This dipole moment in turn drives holes (electrons) toward (away) from the surface and increases the photoluminescence efficiency. Calculated electronic structure and local density of states also reflect a large charge accumulation near adsorbed S atoms.